Many drugs are supplied in rubber stoppered vials in the form of dry powders or concentrates which have to be dissolved (reconstituted) or diluted with a fluid before use. In the past, the material in such vials has been reconstituted by inserting a conventional hypodermic needle through the rubber stopper and adding fluid. However, as this fluid fills the vial, air within the vial is compressed causing a pressure buildup. This may cause several problems. For example, when the needle used to add the fluid is withdrawn, small drops of the reconstituted solution may escape. If the reconstituted solution is one which may cause allergies or local skin reactions, the escape of only a few drops onto the doctor's or nurses hand may be undesirable. Also, in withdrawing samples from the vial, care must be taken that the pressure does not force the plunger of a unit dose syringe rearward so as to cause an overdose in the syringe.